The present invention is directed to an attenuator arrangement, particularly a PIN diode attenuator arrangement intended for the SHF frequency range, with which arrangement it is possible to improve the control characteristics of the attenuator.
In the reception of phase modulated emissions, such as QPSK and QAM, the detector requires a signal which is amplified in linear stages and regulated to a suitable level. The emission level reaching the receiver can change during reception. An automatic gain control (AGC) system is required in order to have a constant signal level at the input of the detector. The control system includes a feedback, which compensates for the amplitude changes of the received emission by changing the gain of the receiver. Usually the control information about the gain is obtained as a voltage. Similarly, integrated circuits having an AGC function are generally voltage controlled. In the control system it is essential that the control loop is stable, and that the gain and phase margins are large enough to enable the compensation for component tolerances and any non-linearity.
At high frequencies a diode is one of the most common components used to control the signal level. Particularly a PIN diode is very suitable to be used in micro-wave circuits. It is used i.e. as a rectifier, a modulator, an attenuator, a switch, a phase shifter, and as a limiter. Here we examine the use of a PIN diode in attenuator applications.
A PIN diode attenuator can be controlled by regulating the direct current flowing through it. In a diode attenuator comprising PIN diodes the diodes can be connected in the signal path, either in series or in parallel. At high frequencies the maximum attenuation in a series connection is limited by the diode capacitance. Correspondingly, in a parallel connection the maximum attenuation is limited by the diode inductance, including the encapsulation. At high frequencies a parallel connection is restricted by, in addition to the inductance, the stray inductance of the capacitor required by the biasing connections. The capacitor can be dimensioned so that the dimensioning also observes the created stray inductance, whereby the capacitor can be brought into series resonance at the used frequency. This creates a so-called notch or trap circuit. However, when the frequency range is wide this method does not work, because the obtainable attenuation is strongly frequency dependent.
It is also possible to connect PIN diodes both in series and in parallel, whereby the obtainable maximum attenuation decreases rapidly when the frequency increases due to the above mentioned reasons. In wide band applications this is not desirable. An essentially more even frequency response than that of the previous configuration is obtained if there are used diodes connected in series and if the load impedance of every single diode is capacitive.
When a PIN diode acts as a series element the maximum attenuation depends on the diode capacitance in the reverse blocking state, and the minimum attenuation depends on the diode impedance at the highest current. The load impedance affects both the maximum and the minimum attenuation. Due to the diode capacitance at radio frequencies the attenuator""s control range may be rather narrow, whereby it may be necessary to use an attenuator series connection containing several diodes in order to realise the required control range. FIG. 1 shows a prior art attenuator arrangement.
The attenuator arrangement presented in FIG. 1 comprises three series connected PIN diodes D1, D2, D3. The diodes are biased with the biasing voltage UBIAS via the biasing circuit comprising a capacitor CB and a resistor RB. The operation of the attenuator is controlled with the control voltage UAGC via the resistor RG. The impedances Z1, Z2, Z3 represent the load impedances of the diodes. The load impedances Z1, Z2 and Z3 are not necessarily physical components, and therefore they are separated with a dotted line from the circuit shown in FIG. 1. It is also possible to add the impedances as physical components to the attenuator circuit, whereby a better voltage distribution between the diodes can be obtained. Mainly at high frequencies the impedances Z1, Z2 and Z3 represent interferences such as stray capacitances and stray inductances generated in the components of the attenuator circuit. The capacitors C1 and C2 act as decoupling capacitors which decouple the DC voltage used for biasing purposes from the other circuits.
In a known way the current passing through a diode increases exponentially as a function of the voltage. The capacitance of the diode""s equivalent circuit limits the attenuation range, whereby the voltage change over the diode terminals representing the useful attenuation range is quite small. As a result of this the diode voltage is close to the so-called threshold voltage. When several attenuators are connected in series, such as in the arrangement according to the FIG. 1, the same biasing current passes through all diodes D1, D2, D3, whereby the voltage of all diodes is close to the threshold voltage. The resistance of the biasing resistor RB of the diode or diodes D1, D2, D3 must be low in order to have a sufficient current for decreasing the minimum attenuation of the attenuator. When the resistance RB is low the control range of the control voltage UAGC corresponding to the attenuator""s operation is limited to a small part of the total range of the control voltage. In practice this means that the change dG/dU of the attenuator""s gain is not constant but depends strongly on the value of the voltage UAGC. Due to this there is a tendency in the AGC system towards instability, which, depending on the level, changes among other things the settling time of the voltage UAGC.
The object of this invention is to present a method for biasing a diode attenuator where the stability of the AGC system, and in some cases, also the control accuracy can be improved. A further object of the invention is to present an attenuator circuit where the biasing method according to the invention is applied.
The objects of the invention are attained by connecting biasing members in connection with the attenuator members, whereby a part of the biasing current can be supplied via the biasing members, whereby in an attenuation situation the attenuator members begin to conduct separately, controlled by the AGC circuit.
The biasing method according to the invention is characterized in that the biasing current of at least one attenuator member is controlled to have a different magnitude than the biasing current of at least one other attenuator member. The attenuator circuit according to the invention is characterized in that at least one biasing member is connected in connection with at least one attenuator member in order to control the biasing current of said attenuator member to have a different magnitude than the biasing current of at least one other attenuator member. Other advantageous embodiments of the invention are presented in the dependent claims.
The biasing arrangement according to the invention is substantially simpler than corresponding prior art solutions. With the solution according to the invention it is further possible to improve the control characteristics of an attenuator based on PIN diodes, because the arrangement according to the invention can improve the stability of the control system and decrease the change in the control steepness caused by the change in the control voltage UAGC.